This invention relates to an electronic switching power supply for supplying power to a load from an input voltage source, including a flyback converter with a transformer having its primary coil connected in series with the collector-emitter circuit of a first transistor in parallel arrangement with the input voltage source, while its secondary coil is connected in series with the load and a first diode, with the emitter of the first transistor being connected to chassis or reference potential through a first resistor while the base of the first transistor is connected to a control circuit, with a capacitor in parallel arrangement with the load, with the voltage across the junction of the capacitor at the side remote from reference potential being of the same polarity as the voltage across the junction of the first resistor and the first transistor.
Electronic switching power supplies are used for delivering a constant voltage and/or a constant current to electrical or electronic devices, being frequently constructed as primary or secondary switched-mode flyback or forward converters or as push-pull converters. As a rule, they include a rectifier circuit with a subsequent filtering and smoothing arrangement to which a transformer is connected. An electronic switch is provided which is connected in series with the primary coil of the transformer and is turned ON and OFF in dependence upon one or several control variables, with the energy stored in the transformer being delivered to an electrical load through a diode of suitable polarity during the reverse period where a flyback converter is used, during the forward period where a forward converter is used, and during both the reverse and the forward period where a push-pull converter is used.
A switching power supply of the type initially referred to is known, for example, from EP 0 130 411 B1. The embodiment shown in FIG. 2 of this specification incorporates a self-oscillating flyback converter, that is, the control circuit connected to the base of the first transistor is substantially comprised of the feedback from the secondary circuit through a capacitor and a resistor to the base of the first transistor, as well as a second transistor having its collector-emitter circuit between the base of the first transistor and reference potential, its base receiving the voltage drop across the first resistor. However, the present invention being not limited to a switching power supply with a self-oscillating flyback converter, it also finds application in switching power supplies having a master-excited flyback converter. In such an arrangement, the control circuit comprises, for example, a multivibrator whose clock frequency or pulse/no-pulse ratio driving the base of the first transistor varies in dependence upon the input voltage, the primary current, and/or the secondary current.
Depending on the application, such switching power supplies are conventionally designed for supplying a predetermined (maximum) operating current. In some applications, however, the load which may be, for example, the motor of a small electrical appliance such as a shaver or a hair-removing appliance, requires a starting current substantially higher than its operating current. Under such conditions, it would be only because of the temporary need for a higher starting current that the switching power supply would have to be designed for peak currents substantially higher than required by the load in operation, accordingly resulting in an increased power dissipation also on termination of the startup phase because of the higher primary peak current. For example, the first resistor arranged in the primary circuit would have to be substantially smaller to enable a higher primary peak current to flow, which would above all produce higher losses in the switching transistor.